Testing the Kerr Black Hole Hypothesis with GX 339-4 by a combined analysis of its thermal spectrum and reflection features

TL;DR

This study tests the Kerr black hole hypothesis using simultaneous NuSTAR and Swift observations of GX 339-4, analyzing thermal and reflection features with advanced models to measure deviations from Kerr geometry.

Contribution

It provides the first precise constraints on the Johannsen deformation parameter using combined thermal and reflection spectral analysis of a black hole binary.

Findings

Measured $oldsymbol{ ext{α}_{13}}$ close to zero, consistent with Kerr black hole.

Achieved unprecedented precision in testing deviations from Kerr geometry.

Assessed systematic uncertainties with different spectral models.

Abstract

We analyze simultaneous observations with NuSTAR and Swift of the black hole binary GX 339-4 in which we clearly detect both a strong thermal component and prominent relativistic reflection features. We employ NKBB and RELXILL_NK, which are the state of the art of, respectively, the thermal and reflection models to test the Kerr black hole hypothesis. We obtain unprecedented precise measurements of the Johannsen deformation parameter $\alpha_{13}$: $\alpha_{13} = -0.012_{-0.039}^{+0.011}$ (lamppost coronal geometry) and $\alpha_{13} = -0.010_{-0.018}^{+0.024}$ (broken power-law emissivity profile) at a 90% confidence level, where the Kerr metric corresponds to $\alpha_{13} = 0$. We investigate the systematic uncertainties by fitting the data with different models.